Reactions of co-ordinated nitriles. Part I. Reactions of cis-(aminoacetonitrile)bis(ethylenediamine)halogenocobalt(III) complexes in aqueous solution

Abstract

In near neutral or basic solution, the ions [Co(en)₂X(NH₂CH₂CN)]²⁺(X = Cl or Br; en = ethylenediamine) undergo rapid base-catalysed ring closure to give purple [Co(en){NH₂·CH₂·C(NH₂):N·CH₂·CH₂·NH₂}X]²⁺, (I), containing a tridentate amidine ligand. The kinetics of this process have been studied spectrophotometrically at 25 °C and I= 0·1 M in the pH range 5·9–7·0 (X = Cl, k_OH= 2·57 × 10⁴ l mol^–1 s^–1; X = Br, k_OH= 4·50 × 10⁴ l mol^–1 s^–1). Condensation in the ring-closed product (I) occurs at the nitrogen centre trans to the halide ion. Base hydrolysis of the halogeno-ligands in (I; X = Cl or Br) has been studied, both spectrophotometrically and by pH-stat at 25 °C and I= 0·1 M. The two methods give rate constants in excellent agreement, k^Cl_OH= 2·44, k^Br_OH= 1·18 × 10¹ l mol^–1 s^–1. The Hg^II catalysed aquation of (I; X = Cl or Br) has also been investigated. Both base hydrolysis and mercury(II)-catalysed aquation appear to occur with essentially complete retention of configuration. The mercury(II)-catalysed reactions of the chloro- and bromo-complexes[Co(en)₂X(NH₂CH₂CN)]²⁺ in 0·537 M-HClO₄ have been investigated. The product of these reactions is the chelated glycine amide complex [Co(en)₂(NH₂·CH₂·CO·NH₂)]³⁺, and the mechanism of this reaction is discussed.